The present invention relates to aircraft control systems and techniques and more particularly to a system for controlling the ground speed of an aircraft to produce hovering of the aircraft.
In aircraft control systems of the prior art, it has often been desirable to reduce ground speed to zero during flight to permit aircraft hovering, particularly in helicopter and VTOL aircraft. In order to accomplish aircraft hovering, a measurement of ground speed is first made using a unit such as a Doppler radar or inertial system to specifically measure the ground speed. The value for ground speed is then coupled to the flight control system in the aircraft which reduces the velocity of the aircraft until the measured value for ground speed equals zero. Normally this is accomplished by controlling the trim angle on the rotor of the helicopter, for example, to change its pitch attitude and thereby controlling its speed. In systems of this type, the pitch attitude is changed until the helicopter ground speed is brought to zero, thereby maintaining a hover condition.
One of the disadvantages of such prior art systems has been the cost required to include apparatus for directly measuring aircraft ground speed. Doppler radar, for example, may make the required ground speed measurements but is costly and also significantly increases the weight and space requirements of the aircraft. Inertial systems likewise provide a direct measurement of ground speed but are also expensive and heavy and decrease the versatility of the aircraft and its use in certain environments. In new generation helicopters and VTOL aircraft, space, weight and cost considerations are critical to the effective marketing and operation in their intended fields.
In spite of the above deficiencies, there is still a great need in helicopter and VTOL aircraft to include measurements of ground speed in order to allow automatic control of present day aircraft. There is also a continuing effort to provide completely automatic aircraft control by use of an autopilot for automatic takeoff, landing and hovering of the aircraft. Since some measurement related to groundspeed is necesary to produce the control required to achieve hover, there has been no alternative but to provide expensive and complex equipment to make that measurement. There is therefore a need for systems and techniques which allow an aircraft to achieve a hover condition for landing in restricted environments.
The present invention has been developed to provide a technique for enabling hover without a direct measurement of ground speed and to overcome the limitations of the above known and similar techniques.